The instant invention relates to a circuit for automatically shutting down a lamp and more particularly to such a circuit which automatically shuts down the lamp in an electrophotocopying machine after the lamp has been on for a predetermined amount of time.
In facsimile systems or in office copy machines, it is customary to have relative motion between an original document and a source of scanning rays. The scanning rays are scanned in some predetermined fashion across the document for subsequent utilization thereof, i.e., transmission or recreation of the original document. The brighter the image scanning rays become the more responsive and selective may be the detection system. With a low powered source of image scanning rays, the detection system must of necessity be more sensitive, which inherently adds noise or other undesirable effects to the output signal therefrom.
It is therefore desirable to use as high an energy exposure lamp as is possible without causing damage to the original document or the glass platen supporting the original document. Such a high powered source of rays is desirable in that information detection becomes less complex with the additional advantage of higher resolution and information density. If, however, a paper jam occurs in the machine, whether a facsimile scanning device or office copying machine, the danger posed by the extreme heat radiated by the high energy exposure lamp increases. As an original document may be difficult to replace, it is highly desirable to deenergize the exposure lamp and/or the transport system as quickly as the jam is detected so as to prevent scorching, burning or tearing of the original document or damage to the glass platen.
One prior art solution to this situation is to continually monitor by mechanical means the movement of the document through the mechanical transport. Such a system is undesirable, however, as different size paper or torn or mutilated sheets may be used which render the mechanical detection device inoperative. If such a jam would occur with an unreliable jam detector, the possiblilty arises of the original document being destroyed due to the heat energy radiated by the high energy exposure lamp or tearing by continual movement of the transport system.
Another prior art solution is disclosed in U.S. Pat. No. 3,600,610, issued Aug. 17, 1971, which utilizes rollers and a cam to open and close a microswitch to thereby energize a timing circuit which allows the lamp to continuously illiminate the document. If the paper jams in any manner, the rollers stop since the paper itself is not moving through the rollers, and the cam is no longer able to operate the microswitch and after a predetermined time delay the timing circuit will deenergize the lamp. The rollers and cam are in effect a sensing device, and the instant invention provides a circuit for deenergizing a lamp which does not require any sensing devices and utilizes fewer components than any of the prior art solutions.